1. Field of the Invention
The present invention generally relates to a filtration system for removing particulate material from a gas stream, and in particular, to a filtration system which conditions the gas stream at an intermediate point along the filtration process to better enable downstream removal of particulate material.
2. Background of the Invention
In the past, systems have been proposed for removing particulate material from an air stream during a manufacturing process. One such environment exists in the wood manufacturing industry wherein particle board, medium-density fiber board, oriented strand board and the like are formed. To produce these wood products, wood chips and wood particles are combined to form a solid panel. During the manufacturing process, the wood particles and chips are passed through a dryer which removes moisture therefrom. Once dried, the wood chips and particles are conveyed, such as within a pneumatic conveyor, to a former and press which cooperate to compress the wood particles and chips into a desired end product, such as a board, panel, or sheet. Additionally, in the wood manufacturing industry, the dryer discharges several gaseous compounds, such as the moisture removed from the wooden particles and chips, gaseous tar, terpenes and the like. Examples of these gaseous compounds include beta pinene, limonine, camphene, alpha pinene, and the like (hereafter collectively referred to as terpenes).
As is known in the industry, the raw material (i.e., the wood particles and chips) may be delivered to and removed from the dryer via pneumatic conveyors. However, the raw material may not be delivered directly to the forming process via a pneumatic conveyor. Instead, the raw material must be removed from the air stream within which it is entrained during the conveyance process, prior to being delivered to the forming process.
Conventional systems utilize particulate separators, such as cyclones, to remove the entrained raw material from the air stream and to deliver the raw material to the forming process, while exhausting the air stream to the atmosphere. The gaseous compounds and the liquid vapor discharged from the dryer are similarly discharged within the gas stream output from the separator. However, the separators used within conventional systems are less than 100% efficient and thus discharge a percentage of the raw material into the atmosphere. Such discharge is undesirable as it wastes raw material and damages the environment.
It should be understood that the term "separator", as used throughout this application, is intended to refer to any conventional separation device, known within the industry, to remove particulate material from the gas stream emitted from a wood dryer. These separation devices remove approximately 80-95% of the wood particulate material from the gas stream.
In the past, a secondary particulate removal device has been provided downstream of the primary separator to further remove particulate material from the gas stream. However, these secondary removal devices, such as multi-clones, still leave an undesirable percentage of the wood particulate material within the gas stream. In some instances, a thermal oxidizer has also been included downstream of the conventional secondary removal devices to remove the VOCs from the gas stream. However, in order for the thermal oxidizer to remove VOCs effectively, it must receive a gas stream substantially void of particulate material. Hence, conventional systems have been unable to effectively use thermal oxidizers since such systems have been unable to remove substantially all of the particulate material from the gas stream.
The conventional secondary particulate removal devices have been limited to the types of devices, such as multi-clones, which leave approximately 5% of the wood material within the gas stream. More efficient removal devices have proven ineffective within the wood drying industry. For instance, fabric type particle collectors have proven ineffective when installed upstream of the thermal oxidizer since the fabric type particle collector blinds and plugs with the particulate material. The particulate material blinds and plugs the fabric due, in part, to the presence of the gaseous compounds and the liquid vapor discharged from the dryer. Once discharged from the dryer, the gaseous compounds and liquid vapor begin to cool as they are transferred through the equipment and along the ductwork. Such cooling continues until reaching a temperature corresponding to the dew point of the compound and liquid. When the temperature reaches the dew point, the compounds and liquid vapor condense. Typically, this condensation occurs upon the filter media within the fabric collector. As the particulate material contacts the condensation, it blinds and plugs the filter.
Additional inefficiencies of a fabric collector type separator result in conventional wood filtration systems due to the low percentage of particulate material entrained within the gas stream delivered to the collector. Approximately 90% of the particulate content of the incoming air stream is removed in the primary separator. The remaining percentage of particulate material is too small to enable the fabric collector to establish a consistent "dust cake" about the exterior of the filter media. The term "dust cake" is a term of art which refers to the phenomenon whereby filtered particulate material forms a collective layer about the fabric media (such as a filter bag within a baghouse). The collective layer protects the filter media from blinding over or plugging. The dust cake increases the filtration efficiency of the filter media. However, the above-noted fabric-type collectors have proven ineffective when used with a wood dryer, since insufficient dust is present to form a dust cake. Moreover, the condensation, within the collector, of vapor and gaseous compounds has further interfered with the formation of a dust cake upon the fabric media.
An alternative secondary collection system has been proposed which utilizes a wet filter wherein the gas stream is passed through a chamber containing liquid. The particulate material collects within the liquid and is removed from the air stream. However, such wet filtration systems produce a slurry byproduct of liquid and particulate material which is environmentally disadvantageous. Also, such wet type filtration equipment is overly expensive to achieve the desired filtration efficiencies and volume capabilities.
A need remains within the industry for an improved filtration system for use with wood dryers. It is an object of the present invention to meet this need.